The present invention relates to starter modules for discharge lamps, especially for motor vehicles.
This discussion of the background is illustrated in FIG. 1 of the attached drawings, a brief description of which appears later in this specification. As shown in FIG. 1, a conventional starter module for a discharge lamp comprises a transformer 1 having a secondary winding 1a connected in series with the discharge lamp 3. The primary winding 1b of the transformer is connected in series with a switching element 2, such as an IGBT thyristor interruptor. A capacitor 4 is connected in parallel with the circuit branch which includes the interrupter 2 and the primary winding 1b. 
During starting of the lamp, the interruptor 2 and the transformer 1 cause a high tension starting pulse, of the order of 12 to 25 kV, to be generated between the electrodes of the discharge lamp. Then, and in particular in normal operation, the transformer 1 plays no part in the operation of the discharge lamp, except through its secondary winding 1a which acts as a self-inductance.
The transformers of starter modules employed for discharge lamps in motor vehicle headlights are generally so dimensioned, electrically and mechanically, that their secondary winding 1a has a high saturation level, i.e. an intensity of saturation current which is in general greater than 4 amperes.
It is however desirable to be able to miniaturise the starter modules and their transformers so that they can be incorporated either in the discharge lamp bases or in connecters which are adapted to receive the bases of the discharge lamps, or again, in a compact electronic module which provides all or some of the functions of the control circuit (i.e. a so-called ballast circuit) in which these latter may or may not be incorporated.
Miniaturisation of a starter module transformer involves the need to have much reduced saturation levels (of the order of 1.5 amperes) for the secondary winding. This results in very high current peaks, in excess of 20 amperes, which are due in particular to the transitions generated by starting of the lamp during the lighting-up phase.
These high current peaks pose a dimensioning problem for the various components of the ballast, that is to say for the components of the power supply circuit other than those in the starter circuit.
An object of the invention is to overcome the above problem.
According to the invention in a first aspect, a starter module for a discharge lamp, especially for a motor vehicle, including a high tension transformer which includes a magnetic circuit on which a primary winding and secondary winding are wound, is characterised in that it further includes means for modifying the magnetic characteristics of the transformer so as to depress the saturation level of the secondary winding.
Preferably, the said modifying means comprise means for providing complementary magnetisation on the transformer. In that case, the said modifying means comprise a permanent magnet disposed in the magnetic circuit. Preferably then, the permanent magnet is disposed in an air gap of the magnetic circuit.
The permanent magnet preferably includes indexing means to ensure its correct orientation.
In the case where the means for modifying the magnetic characteristics of the transformer comprise a complementary magnetising means, the complementary magnetising means comprise a third winding which is wound on the magnetic circuit and which is energised continuously or transiently.
The magnetic circuit preferably comprises an annular core.
According to the invention in a second aspect, a discharge lamp for a motor vehicle is characterised in that its base incorporates a starter module according to the invention.
According to a third aspect of the invention, a connecter for a discharge lamp for a motor vehicle is characterised in that it incorporates a starter module according to the invention.
According to the invention in a fourth aspect, an electronic module which is adapted to provide all or some of the control functions for the operation of a discharge lamp is characterised in that it includes a starter module according to the invention.
Further features and advantages of the invention will appear more clearly on a reading of the following description of some preferred embodiments of the invention, which is given by way of non-limiting example only and with reference to the accompanying drawings.